1. Brief Description of the Related Art
Private communications networks permit the circulation of information within the same building or between different buildings. These buildings can e.g. be the various branches of a bank or the different rooms in a factory.
The digitally encoded information travels between stations distributed in the buildings and in each case comprise an emitter and a receiver. The information transmission supports are optical fibres in which each transmission channel corresponds to a particular wavelength. Each optical fibre permits the multiplex propagation of several light beams of different wavelengths.
Such optical communications networks are known and in exemplified manner reference is made to the network designated under the name Lambdanet developed by ATT-Bellcore or the Multicolore network developed by the Centre National d'Etude des Telecommunications (CNET).
The present invention relates to another private network type which, instead of using optical multiplexing, uses coherence modulation, or in other words the introduction of an optical delay modulated by a time exceeding the coherence time of the light beam, which supports the information.
Coherence modulation is known per se and is described in French Patent 2 608 869. An information is encoded by delaying part of the light beam by a main delay greater than the light beam coherence time and to which is added a variable modulation delay representing the information bits to be encoded. The variable delay is obviously very small compared with the fixed delay and consequently below the coherence time.
In order to decode the information, interference is brought about between the parts of the light beam by delaying the part in advance by the same main delay. This gives an intensity modulation corresponding to the variations of the variable delay, i.e. to the encoded information bits.
An advantage of a private network using coherence modulation is that it is possible to use several carriers (each carrier corresponding to a given main delay) on the same support, whilst ensuring a total separation of the different channels on reception.